Guanajuato Build

[Jubilado]

I've studied this forum and now ready to begin my project by soliciting input on a few topics that are still unclear. I find this forum incredibly helpful as I seem to learn something quite useful each time I search. My priority now is to finalize design and order materials.
My build is a 90 cm Pompeii (hemispheric) design. The oven will be used for pizza, of course, and some forays into wfo baking and meal preparation. Post-vaccine get-togethers.
I'll start seeking help with some loose ends I need to tie before ordering materials. I hope that some of you experienced oven builders will comment.
Brick Layout
I can construct the radius with a 1:1 combination of square-cut half brick and available arch brick. I would set these so that there is no mortar on the inside and a tight joint on the outside.
With the taper accomplished in one dimension, I can lay out the bricks from a top view so that only one cut is necessary for a tight fit. This will create a saw tooth pattern that will not be pronounced until the upper brick courses. I don't use 3-D cad, but I think I have this right. This will result with a tight joint and avoid four additional cuts.
Insulation
The oven will be backed into a corner adjacent to the house walls. Space for insulation is tight, which demands very low conductivity insulation material.
I am considering flexible rock wool or perhaps high temperature fiberglass after 2" of ceramic fiber for the dome. I would like to avoid vermicrete with the cure-out issues and higher thermal conductivity.
I have allowed for 8" insulation between the oven floor and the existing supporting conventional reinforced concrete slab. My thermal calculations show a pretty high temperature coming through into the reinforced concrete. The 8" should give me around 50C at the bottom of the slab. Maybe overkill, but I see in the forum difficulty in maintaining oven floor temperature.
I haven't got my hands on a few board insulation types that I am considering (for low conductivity). Has anyone had experience with ceramic fiber board or micro-porous board? I would place these beneath calcium silicate, as the calcium silicate is much stronger.
Thanks for reading and considering.

[UtahBeehiver]

In the pdfs, they only show 2 dimensions, what you do not see is how the bricks need to lay in 3d in order to have tight joints and eliminate what is known as "inverted V" joints. On paper always is easier than real like, True mortarless is a very difficult process to do without lots of time, a good wet saw (12" and bigger) and ability to do compound cuts. I am not sure it is worth the effort. Than said you can get really tight inside joints with "some" beveling and not much tapering (angle) refer to JR PIzza's build.

[UtahBeehiver]

8" of floor insulation is probably over kill and especially CaSi or equivalent. CaSi is expensive and there are diminishing returns of using too much. If it is p or v crete 5 to 1 on floor then 6" would work fine.

[Boogie-D]

Jumping in here cause I am also planning to begin my build... I’ll be doing the perlicrete floor on top of concrete table... russel are you suggesting 6 inches of CaSi over the perlicrete?? Thanks

[Jubilado]

Russell,

Thanks for looking, your analysis, and your sketch.

I am not looking to achieve the mortarless grail, but rather limit the number of cuts needed to get minimal mortar, perhaps none inside the dome. I believe some of that is accomplished as shown in the pair of 2D sketches, but it needs a 3D layout to accurately determine. If I can't find drafters with the CAD software here locally I'll be making cardboard bricks to mock up. A good covid pastime, I suppose.

Rob

[UtahBeehiver]

As I mentioned, look at JR Pizza's build, min cuts, done in CAD (by him), tight inner joints. no inverted V

[Jubilado]

I have been wrestling with the thermal design of the oven. I have a fair amount of experience with furnaces in the industrial universe.

My calculations show that the hearth would transfer too much heat with most of the insulation scenarios I find here. I have been trying to reconcile the discrepancy. Just two days ago I saw the thermal analysis that Karangi Dude performed on his 48", which shows lower temperatures than my calculations. The thermal calculations often are pretty accurate, but only for equilibrium. My thought now is that these ovens never actually achieve equilibrium - it would take a very, very long fire - always heating up or cooling down.

So my estimates of how much insulation have been very conservative. I am working from a 36" high installed concrete base, I want a high cooking surface, so there is space to fill. I would prefer dry insulation, rather than insulating concrete, to avoid the long cure and the relatively mediocre insulating properties. I may be forced to use ceramic board as so far I am unable to find calcium silicate board here.

Oh, the calculation summary. Anyone please feel free to audit.

HEARTH
	Thickness		k	T
	In	m	W/m°K	°C
Inside wall	-	-	-	400
Brick	2.5	0.064	1.20	390
Cal Sil	2	0.051	0.10	298
perlcrete 6:1	1.5	0.038	0.14	248
perlcrete 6:1	2	0.051	0.13	177
perlcrete 6:1	2	0.051	0.12	100
concrete	3.5	0.089	1.10	85.7
Air temp.			20	°C
h - Still air, upward surface			2.77	W/m2/°C
Q/A			182	W/m2/h
Q			116	Watts/h

Rob

[Jubilado]

The model for the calculation is off in a couple of respects. The heat in the slab will flow horizontally from beneath the oven, lowering the slab temperature and increasing the heat loss. Two dimensional analysis is cumbersome unless proper software is available. Also, the air under the slab will heat up, raising the temperatures and lessening the heat loss.

[david s]

I've studied this forum and now ready to begin my project by soliciting input on a few topics that are still unclear. I find this forum incredibly helpful as I seem to learn something quite useful each time I search. My priority now is to finalize design and order materials.
My build is a 90 cm Pompeii (hemispheric) design. The oven will be used for pizza, of course, and some forays into wfo baking and meal preparation. Post-vaccine get-togethers.
I'll start seeking help with some loose ends I need to tie before ordering materials. I hope that some of you experienced oven builders will comment.
Brick Layout
I can construct the radius with a 1:1 combination of square-cut half brick and available arch brick. I would set these so that there is no mortar on the inside and a tight joint on the outside.
With the taper accomplished in one dimension, I can lay out the bricks from a top view so that only one cut is necessary for a tight fit. This will create a saw tooth pattern that will not be pronounced until the upper brick courses. I don't use 3-D cad, but I think I have this right. This will result with a tight joint and avoid four additional cuts.
Insulation
The oven will be backed into a corner adjacent to the house walls. Space for insulation is tight, which demands very low conductivity insulation material.
I am considering flexible rock wool or perhaps high temperature fiberglass after 2" of ceramic fiber for the dome. I would like to avoid vermicrete with the cure-out issues and higher thermal conductivity.
I have allowed for 8" insulation between the oven floor and the existing supporting conventional reinforced concrete slab. My thermal calculations show a pretty high temperature coming through into the reinforced concrete. The 8" should give me around 50C at the bottom of the slab. Maybe overkill, but I see in the forum difficulty in maintaining oven floor temperature.
I haven't got my hands on a few board insulation types that I am considering (for low conductivity). Has anyone had experience with ceramic fiber board or micro-porous board? I would place these beneath calcium silicate, as the calcium silicate is much stronger.
Thanks for reading and considering.

Pretty much all refractory suppliers should be able to supply cal sil board. You may also like to consider foam glass as it doesn't absorb moisture.

Be careful if planning a corner build as working the outside layers and outer render can be extremely difficult unless you leave lots of room. The other problem is that it creates an area behind the oven in the corner that you pretty much can't use for anything.

Fibreglass or rock wool although being cheaper than ceramic fibre blanket, don't have sufficient density to work over with subsequent layers without the material compressing. I tried rock wool on two ovens but the difficulties it created weren't worth the cost saving. https://community.fornobravo.com/for...872#post433872

[Jubilado]

After prodding from the forum (thank you, beehiver), rekindling 10th grade geometry in my old brain, and locating a few more posts describing the inverted vee, I understand how it arises. This is not an issue to me, though, quite acceptable to fill the gap.

My oven walls will be a half-brick thick. If my logic and layout are correct, I can make the half-cut on a slight angle, rather than square, and have thinner mortar joints, inside and out. This seems feasible at least for the first several courses. And ditto in the vertical direction by alternating courses of standard and tapered bricks. As a newbie, I'm just nervous about when the construction starts and grateful for comments from the forum.

Not finding cal sil after contacting about 5 supply houses is frustrating. Material availability may be my worst problem. It seems that ovens here are set on ceramic fiber board, but I don't feel good about uncharted materials in the foundation.

[Jubilado]

I am still designing and sourcing materials and prices. I've still got more to finalize before purchasing materials.

I was looking at Foamglas for underneath oven insulation. I could go with 6" foamglas or foamglas covered by calsil. Does anyone have the experience to know if it is ok to place refractory brick right on top of Foamglas? I expect Foamglas could be easily abraded; although it has a compressive strength of 60 psi. I am planning just placing the brick in place on the 4-1/2x9 surface, unattached, separate from the dome structure.

[Jubilado]

My design is fairly complete, so I am posting some drawings for comment. We will be heading off to buy materials and tools next week in the US.

I cannot get my hands on calsil. It's not available here and I am running into large minimum order issues in the US. If anyone knows of a good source in San Antonio area, please share. Right now I am planning to substitute ceramic fiber board as a top layer over foamglas. I believe the brick could not be placed directly onto foamglas.

Info for the SK34 brick and the ceramic fiber board are attached. Hopefully the data will overcome any Spanish language issues. The brick seems ok to me, but there are many similar specifications I've seen that show little difference. The ceramic fiber board seems a bit wimpy, so I am wondering if anyone has used similar material beneath the oven.

A 2" thermal break entirely separates the oven from the landing. The chimney will rest on the landing arch with additional lateral support from the roof structure to be above. This will give me a bit more landing and better thermal performance. A 6" square stainless flue will have a curved landing for the arch mount and has an area somewhat larger than the recommended 6" diameter. Landing opening is slightly flared.

Comments are very welcome.

Rob

[Blairt]

Hi Jubilado. I'm curious which specific Foamglas you're using. Looking at the Owens Corning website, they have 4 versions. How did you arrive at the 6" thickness? It seems like overkill when others suggest 4" insulation under the oven base.

Like you, I'm finding it difficult to source some products - specifically, ceramic board. I'm considering Foamglas now and am curious on what others think of it.

[Jubilado]

My Foamglas inquiries have not included the type - only the block form. I have observed that the manufacturer's sites are confusing as to the different types. I get the impression that this material is used most often for pipe, perhaps cryogenic. I don't think that there is enough difference in the types to matter for brick oven support.

In Mexico Foamglas is very difficult to find. Many of the insulation companies list it on their website, but they don't actually carry it. I think I have found one supplier though. In the US there are more suppliers, but those I found are wholesalers that sell in box quantities. The block form is 18" x 24". Box quantities seem to fill the same size box: 12 pieces of 2", 8 pieces of 3". Distribution International sells it, but it is not stocked in all their locations. Shipping is expensive - because it is fragile, they prefer to ship via motor freight.

I can find ceramic fiber board here. It seems that is what is commonly used in Mexico. The compressive strength is low, but it really just needs to be tough enough to hold up under the brick. I figure that compression loads from the oven are less than 4 psi. I hope to get my hands on some just to inspect. It would be nice if it has sufficient surface toughness. I have seen higher density, stronger fiber boards, like transite, but the thermal performance is lower and it comes in 4'x8' sheets. Calcium silicate is as rare as foamglas in Mexico, so I'll likely wind up with the insulating ceramic fiber board.

As far as insulation thickness under the oven, from calculations 4" seems very thin to me. On this form I see, though, that this is often done, with an occasional complaint about poor oven floor temperatures. I also see 2" calcium silicate on top of 5-1/2" of perlcrete. In my case, I have a 36" high existing platform and I want to raise the oven floor 8"-10" higher. I like using the dry insulation to avoid lengthy cure-out and also get better thermal performance. Perlcrete is inexpensive, but I am less sensitive to the cost than others.

The thermal calculations for the floor are a bit complex, as the thermal gradient goes downward through the insulation and then horizontally through the concrete support slab with the heat transferred by convection to the air. For my last calculations I have assumed the concrete slab area for heat transfer is the area under the oven plus wall insulation. With 6" of foamglas and 1" of calsil (or ~ceramic fiber board) I estimate a surface temperature of around 140F. I know this is conservative, but I like the solution with my resultant oven floor height along with the knowledge that the oven floor temperature and heat loss will be good.

Rob

[UtahBeehiver]

I all comes down to material availability and budget. I did use a combo of FoamGlas (will not absorb water) and CaSi because of my climate and it was available. P or V crete seems to be a more readily available option you just have to use more thickness relative to CaSi or FoamGlas (rule of thumb is twice the thickness). FoamGlas does have a lower max working temp of around 900 F but I do not believe one will see that temp under the floor bricks. Here is a table of P/V crete K values.